[time-nuts] Switching regulator replacement for 7805

Attila Kinali attila at kinali.ch
Sun Dec 4 18:02:05 EST 2016

On Sun, 4 Dec 2016 16:41:13 -0500
John Ackermann N8UR <jra at febo.com> wrote:

> The little OCXO I used for testing goes from about 850ma cold to 200ma 
> hot (at 5V) and I noticed a very different appearance of the spectrum as 
> the current dropped -- at high current, the switching spurs were quite 
> narrow, but as the current dropped (and particularly below some 
> threshold I don't remember) the spurs widened out quite a bit.

This is exactly the PWM to PFM mode switch I described earlier.

The 850mA are probably high enough that the DC/DC converter works
in PWM mode, thus switching with a constant frequency, even if the
load changes. On the other hand 200mA of a 1A converter is low enough
that it's most likely in the PFM region, where every small change
(or noise) causes the switching frequency to change.

			Attila Kinali

Malek's Law:
        Any simple idea will be worded in the most complicated way.

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